(1) Field of the Invention
The present invention relates to optical writing devices that perform optical writing on a photoreceptor by using a light beam, and image forming devices incorporating same.
(2) Related Art
Among image forming devices such as printers are image forming devices that have an optical head that focuses light beams emitted from a plurality of light-emitting elements through an optical lens, for example a rod-lens array, to expose a photoreceptor.
FIG. 39 is a schematic plan view showing positional relationships between light-emitting elements and a rod-lens array, when the light-emitting elements are viewed through the rod lens array from the photoreceptor.
As shown in FIG. 39, light-emitting elements 1 are arranged in a two-dimensional array, i.e., a plurality of light-emitting elements 1 are arranged in a light-emitting element row 901, which is a line along a main scanning direction, and a plurality of light-emitting element rows 901 are arranged along a sub scanning direction. Each of the light-emitting elements 1 emits a light beam towards a rod-lens array 910.
The rod-lens array 910 is an elongate member in which a plurality of rod lenses 911 are arranged in a zigzag pattern along the main scanning direction, each of the rod lenses 911 having a diameter greater than a diameter of one of the light-emitting elements 1. Light beams emitted from the light-emitting elements 1 are transmitted through the rod-lens array 910 and are focused on a photoreceptor surface.
Screening is known as one method of dithering to express a concentration of mid-range values of an image in binary values.
For example, as shown in FIG. 40A, when an image 602 of a uniform mid-range concentration is present on a page area 601 and screening is performed on the image 602, the image 602 is converted into a screening pattern 603 (output image) in which fine dots (black pixels) are arranged at a density corresponding to the concentration, as shown in FIG. 40B.
Specifically, the screening pattern 603 is a pattern image in which a plurality of pixels 9-1, 9-2, . . . , are arranged in a matrix along the main scanning direction and the sub scanning direction, in which filled pixels correspond to black pixels and unfilled pixels correspond to white pixels.
Each of the pixels 9-1, 9-2, . . . along the main scanning direction corresponds one-to-one with light-emitting elements 1-1, 1-2, . . . . By switching a light-emitting element between light-emitting and non-light-emitting (off) states, black and white pixels can be expressed.
In FIG. 40B, the page area 601 indicates an example for which a reference position R in a new image forming device is a write start position in the main scanning direction for writing an image on a photoreceptor.
For example, the write start position, due to execution of an image stabilizing operation for maintaining image quality above a certain level over a long period of time, may be changed from the reference position R when the device is new by a shift in the main scanning direction as required to maintain image quality.
FIG. 40C shows an example of the page area 601 when the write start position has shifted from the reference position R by a distance α in the main scanning direction. According to this example, the page area 601 is formed on the photoreceptor at a position shifted by the distance α in the main scanning direction.
Before and after change in the write start position, if the image 602 is the same, the whole of the screening pattern 603 shown in FIG. 40B can be expressed before and after change as the same gradation expression by using the screening pattern 605 shifted in the main scanning direction by the distance α, as shown in FIG. 40D.
However, the image 602 shown in FIG. 40A and the image 602 shown in FIG. 40C have different write start positions in the main scanning direction.
In other words, the screening pattern 603 shown in FIG. 40B is formed by light-emitting elements 1 in the main scanning direction from the light-emitting element 1-1 at the reference position R. In contrast, the screening pattern 605 shown in FIG. 40D is formed by light-emitting elements 1 in the main scanning direction from the light-emitting element 1-4, which is the fourth light-emitting element from the reference position R.
According to the shift in the write start position in the main scanning direction, pixels 9-4, and 9-5 of the screening pattern 603 are expressed as black pixels by the light-emitting elements 1-4 and 1-5, and pixels 9-4 and 9-5 of the screening pattern 605 are expressed as black pixels by the light-emitting elements 1-7 and 1-8, and therefore the same pattern and the same pixels are expressed by different light-emitting elements for the screening pattern 603 and the screening pattern 605.
FIG. 41A is an enlarged schematic diagram showing examples of shapes of beam spots 3 after light beams pass through the rod-lens array 910 and are focused on a photoreceptor when a plurality of the light-emitting elements 1 each emit a light beam of the same light intensity. FIG. 41B shows examples of waveforms of light intensity distribution of each of the beam spots 3 on the photoreceptor. Here, in FIG. 41A, a high light intensity portion of the beam spots 3 is indicated by a light color and a low light intensity portion is indicated by a dark color.
As indicated in FIG. 41A and FIG. 41B, each of the beam spots 3 on the photoreceptor have slightly different shapes and slightly different light intensity distributions according to where light beams emitted from the light-emitting elements 1 pass through the rod-lens array 910. This is because the rod-lens array 910, due to its structure, has optical properties that make transmittance of light beams different depending on where the light beams pass through the rod-lens array 910.
According to the optical properties of the rod-lens array 910, light emission amounts of each of the light-emitting elements 1 can be corrected by, for example, adjusting current supplied to each of the light-emitting elements 1, but it is difficult to correct the beam spots 3 so that shape and light intensity distribution of each becomes identical.
Accordingly, when, for example, the pixels 9-4 and 9-5 of the screening pattern 603 in FIG. 40B and the pixels 9-4 and 9-5 of the screening pattern 605 in FIG. 40D are expressed as black pixels, differences occur in the exposure amounts of the photoreceptor and in shapes of dots.
Differences occurring in exposure amounts of the photoreceptor means differences occur in concentration and shape of the black pixels before and after the change in write start position.
Thus, even if an input image is the same, there is a problem of variance occurring in concentration and shape of black pixels constituting a screening pattern before and after the change in write start position.
A screening pattern simulates expression of gradation by using only black pixels, i.e., by dot arrangement, and therefore according to the size of the variance in dot concentration and shape, there is a risk of a user perceiving different gradation in a visual comparison of the same input image when the image is expressed by using a screening pattern according to screening processing before the change in write start position and when the image is expressed by using the screening pattern according to screening processing after the change in write start position.
The occurrence of this problem is not limited to when image stabilizing operations are performed, and may occur, for example, when the write start position in the main scanning direction is changed according to a user's instructions.